Adds an acoustic notification when a new user joins the current hosted game.
I use the same sound than `Andy A` proposal in his patch (alarm_warship_03.ogg).
Maybe we can use a new dedicated soundA new option for activate/desactivate the notification.
A new dedicated sound: A ding Bell Tree, creation by [[ https://wildfiregames.com/forum/index.php?/profile/27677-samulis/ | Samulis ]].